This invention concerns improvements in the processing of video image signals to achieve so-called digital video effects.
In such processing, each field or frame of a television image is stored as a series of discrete digital values or pixels containing the luminance or chrominance information of the original signal and obtained by sampling the original signal at an appropriate pixel repetition rate. The original signal may be reproduced by scanning the store, or, by addressing the store in apropriately timed relation to synchronising signals defining the television raster, selected image information can be retrieved from the store and displayed on a television screen in any desired position or orientation. Such retrieved information may be combined with other image information to produce a pattern or collage of discrete images, and by moving the boundaries between respective patterns containing the separate picture information, various effects can be achieved.
When image information from a frame or field of a digitally stored television picture is to be retrieved and reproduced as part of another frame or field of a television picture, with the original picture information being displaced in position and/or orientation, and possibly being reproduced at a different scale from the original image, it is not sufficient to address the store containing the original image with addresses that will simply provide an output signal corresponding to an originally sampled pixel. This is because image information is required at a higher resolution than the pixel repetition rate and thus it is necessary in order to obtain image information relating to points between adjacent pixels, to address the store in order to obtain the individual values relating to a group of adjacent pixels, in an image area including the coordinate position in respect of which image information is desired, and to process the respective values in order to interpolate the corresponding value at the appropriate coordinate position. In known video image processors, devices for performing this operation are referred to as interpolator and may comprise an arrangement of so-called finite impulse response filters, the coefficients of which may be varied by appropriate addressing signals, in order to provide the required output information. In addition, when an original image area is to be reproduced on a smaller scale, or compressed, it is necessary to apply to the filter a so-called compression signal, in order to introduce a bandwidth limitation for the purpose of preventing aliasing errors.
Known systems for the storage and interpolation of digital picture images are relatively complicated and have the disadvantages of high manufacturing cost in relation to the quality of the final picture.